Extender Pigments
Extender pigments have a low refractive index or opacity and therefore do not provide hiding power; they appear transparent in the paint film. It’s a popular misconception that extender pigments are cheap solids or fillers added to reduce the cost of a paint. In fact, the unique properties of certain extender pigments have significant positive effects on various properties of the paint for example, as described further below, mica can improve the water resistance of a film due to its "platy" particle shape and tendency to orient itself in overlapping layers horizontal to the surface.
Barium sulfafate, commonly known as Barytes (BaSO4), is a heavy, naturally occurring material with good stability in acids and alkalis. Barium sulfate pigment is nodular in shape and has lower oil absorption than most other extender pigments in its particle size. This makes Barytes a popular filler for high PVC interior paints and anti-corrosive metal primers. The packing properties of this pigment in paint binders promote good enamel hold-out and low porosity properties. Because it’s mined and ground from natural sources, barytes can vary in brightness or whiteness.fig. (20)
Blanc Fixe is a precipitated barium sulfate (barytes) made by a chemical reaction between barium compounds and sulfuric acid. The major difference between barytes and blanc fixe is the smaller particle size of blanc fixe. A mixture of 75% blanc fixe and 25% alumina hydrate is known as “gloss white” and has been used extensively in the printing ink industry.
Calcium carbonate is one of the widely-used extender pigments in paint. Calcium carbonate is produced naturally from mined materials or synthetically by a precipitation technique. This pigment is produced in many particle sizes from coarse types (used in fillers and as an inexpensive flattening agent for flat latex paints) to fine types (found in gloss and semi-gloss coatings as a spacing extender). Calcium carbonate is soluble in acids but insoluble in solvents, and is relatively non-reactive.fig. (21)
Magnesium silicate is a naturally-mined material commonly known as Talc. Talc provides dry film reinforcement, bridging, anti-settling and anti-sagging properties; it can also improve the sanding properties of primers and undercoats due to the inherent softness of the particle. Exterior coatings containing excessive amounts can show chalking. Talc comes in two main forms: the aciccular type and the lamellar type. Lamellar or platy talcs reduce permeability and improve hold-out, water resistance, and brushing properties. There is some improvement in bridging properties and anti-sagging. fig. (22)
Mica is a relatively non-reactive pigment in a flat, wafer-like disc (lamellar) particle shape. The major source of mica is mineral muscovite ground in either a wet or dry process. Mica is noteworthy for its ability to reduce a paint film’s porosity and impart all the corresponding benefits, such as its water resistance. The lamellar particles orient themselves much like aluminum flakes to re-enforce the film and form an overlapping, scale-like shield in the coating that works well to ‘seal in’ and hide stains, eliminating bleed through. Mica is widely used in primers and sealers in small amounts (as excessive amounts would reduce the adhesion of subsequent finish coatings). It is also used extensively in asphalt roofing mastics, textured paints, and exterior finish paints to reduce checking and cracking.
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